As a class, the primary hydrocarbyl lithium initiators characteristically have been insufficiently reactive to be adequately effective in certain polymerization systems, particularly in some of the solution polymerization of monovinyl-substituted aromatic compounds.
Primary hydrocarbyl lithium initiators are generally feasible to employ and one such, n-butyllithium, is relatively cheap. Yet, their relatively low reactivity has mitigated against their use in some applications. Other types of hydrocarbyl lithium compounds, such as secondary, are adequately reactive generally, yet normally cost more, making them less commercially attractive.
The primary hydrocarbyl lithium initiators, such as n-butyllithium, characteristically are relatively sluggish in performance for the polymerization of monovinyl-aromatic compounds. Thus, a given batch of such an initiator tends to act unevenly with time such that the initiation of the monovinyl-substituted aromatic compound polymerization is spread over a relatively broad time interval. The resultant polymer tends to be a product with a substantial variation in vinyl-aromatic block length or, in the case of homopolymerization, a polymer of relatively broad molecular weight distribution. Other types of alkyllithium initiators, such as secondary, for most purposes do not need reactivity boosting, yet their higher cost and less ready availability mitigates against their use.
Hence, there is a need to utilize the readily available and more economical primary alkyl hydrocarbyl initiators because of their attractive pricing structures wherever possible. Needed are methods to increase the reactivity of the primary hydrocarbyl lithium compounds for the polymerization of monovinyl-substituted aromatic compounds in certain applications, e.g., when narrow molecular weight distribution is desired in polymer blocks or homopolymers of monovinyl-aromatic compounds.